1. Field
The present application relates to a distortion correcting method and a carrier medium carrying distortion correcting program applied to an electronic camera, image processing software, etc. In addition, the present application relates to an optical apparatus provided with an optical system for shooting such as an electronic camera, an interchangeable lens, a camera system, etc.
2. Description of the Related Art
There is known an image processing technique for correcting distortion of an image picked up by an electronic camera (see the following Patent Documents 1 and 2, Non-Patent Document 1, etc.). In the distortion correction, it is necessary that a distortion amount distribution on the image is already known.
When Y0 is the image height at an ideal image point, and Y is the image height at an actual image point, the distortion amount D which a lens for shooting gives to the image is represented by the following expression.D=100×(Y−Y0)/Y0[%]
The distribution of the distortion amount D on the image can be approximated to a function of image height ratio r (=image height/maximum image height) as follows.D(r)≡a1r+a2r2+a3r3+a4r4+a5r5+a6r6+ . . .
It is generally conceived that sufficient good approximation can be obtained even if the number of terms in this function is reduced to 3 in terms of the order of this function as follows.D(r)≡a1r+a2r2+a3r3 
Incidentally, coefficients a1, a2 and a3 in this function vary depending not only on the type (specification) of the lens but also on the lens position (combination of the focal length f and the focal position d) at the shooting time.
In the camera described in Patent Document 1, therefore, a table containing coefficients a1, a2 and a3 for each lens position is provided as distortion correcting information in advance. If the camera refers to this table in accordance with the lens position at the shooting time, the distortion amount distribution on the image can be calculated easily.
On the other hand, Patent Document 2 has disclosed a printer which makes an operator set correction characteristic for distortion correction while displaying an image on a monitor and which executes distortion correction under the setting. According to this printer, the operator can perform desired distortion correction on each image.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-110847    Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-331543    Non-Patent Document 1: Yuto Takahashi, “Lens Designing”, Tokai University Press, p 127, Mar. 20, 1994
In the camera described in Patent Document 1, there is however a possibility that a bad corrected result may be caused by lowering of distortion correction accuracy because accuracy in calculation of the distortion amount distribution on an image picked up at a certain lens position is worsened. To prevent this, the quantity of information in the table must be increased.
On the other hand, in the printer described in Patent Document 2, whether distortion correction is good or bad depends on the operator's sense. Moreover, even a skilled operator can hardly correct distortion perfectly when the distortion amount distribution on the image is complex (for example, when mixed-type distortion aberration occurs). (Here, the term “mixed-type” means a mixture of barrel and pincushion types.) In addition, the distortion correction takes a great deal of labor and time.